The present invention relates generally to application of nickel-titanium alloys to form medical devices. More precisely, the present invention is directed to cold worked nickel-titanium alloys and nickel-titanium clad alloys that have been processed by a deep drawing operation to produce a material exhibiting linear pseudoelastic behavior without a phase transformation or onset of stress-induced martensite that can be manufactured into a medical device.
A focus of developmental work in the treatment of heart disease is an endoprosthetic device referred to as a stent. A stent is a generally cylindrically shaped intravascular device that is implanted in a diseased artery to hold it open. The device is used to maintain the patency of a blood vessel immediately after intravascular treatments, and further reduces the likelihood of restenosis. In some circumstances, a stent can be used as the primary treatment device where it is expanded to dilate a stenosis and then left in place.
A limitation of some prior art stents, especially those of the balloon expandable type, is that they are stiff and inflexible. Often, the expandable type stents are formed from stainless steel alloys and are constructed so that they are expanded beyond their elastic limit. Such stents are permanently deformed beyond their elastic limits in order to hold open a body lumen and to maintain the patency of the body lumen. By the same token, since the material is stressed beyond its elastic limit into the plastic region, the material becomes stiff and inflexible.
There are several commercially available stents that are widely used and generally implanted in the coronary arteries after a PTCA (Percutaneous Transluminal Coronary Angioplasty) procedure, described above. Stents also can be implanted in vessels that are closer to the surface of the body, such as the carotid arteries in the neck or the peripheral arteries and veins in the leg. Because these stents are implanted so close to the surface of the body, they are particularly vulnerable to impact forces that can partially or completely collapse the stent and possibly block fluid flow in the vessel. Under certain conditions, muscle contractions may even cause the stent to partially or totally collapse. Since balloon expandable stents are plastically deformed, once collapsed or crushed, they remain so, possibly blocking or occluding the vessel. These balloon expandable stents, therefore, could possibly pose an undesirable condition to the patient.
Such important applications as mentioned above have prompted stent designers to use superelastic or shape memory alloys in their stents to exploit the self-expanding and elastic properties of these materials. Typically, the superelastic or shape memory alloy of choice is nickel-titanium. Nickel-titanium alloy, commonly referred to as Nitinol, an acronym for Nickel-Titanium Naval Ordinance Laboratory, where it was initially developed, is frequently chosen for use in self-expanding stents and other medical devices due to its highly elastic behavior and resiliency. As a result, a nickel-titanium stent does not deform plastically when deployed, and remains highly resilient inside the body lumen. Because of this resilience, the self-expanding nickel-titanium stent can encounter a deforming impact, yet will return to its initial shape. Therefore, the chance of a permanent collapse of the self-expanding stent due to an impact force is minimized. An example of such shape memory alloy stents is disclosed in, for example, European Patent Application Publication No. EP0873734A2, entitled xe2x80x9cShape Memory Alloy Stent.xe2x80x9d
Near equi-atomic binary nickel-titanium alloys are known to exhibit xe2x80x9cpseudoelasticxe2x80x9d behavior when given certain cold working processes or cold working and heat treatment processes following hot working. Generally speaking, xe2x80x9cpseudoelasticityxe2x80x9d is the capacity of the nickel-titanium alloy to undergo large elastic strains on the order of 8 percent or more when loaded and to substantially fully recover all strain upon removal of the load. Substantially full recovery is typically understood to be less than 0.5 percent unrecovered strain, also known as permanent set or amnesia.
Pseudoelasticity can be further divided into two subcategories: xe2x80x9clinearxe2x80x9d pseudoelasticity and xe2x80x9cnon-linearxe2x80x9d pseudoelasticity. xe2x80x9cNon-linearxe2x80x9d pseudoelasticity is sometimes used by those in the industry synonymously with xe2x80x9csuperelasticity.xe2x80x9d
Linear pseudoelasticity results from cold working only. Non-linear pseudoelasticity results from cold working and subsequent heat treatment. Non-linear pseudoelasticity, in its idealized state, exhibits a relatively flat loading plateau in which a large amount of recoverable strain is possible with very little increase in stress. This flat plateau can be seen in the stress-strain hysteresis curve of the alloy. Linear pseudoelasticity exhibits no such flat plateau. Non-linear pseudoelasticity is known to occur due to a reversible phase transformation from austenite to martensite, the latter more precisely called xe2x80x9cstress-induced martensitexe2x80x9d (SIM). Linear pseudoelasticity has no such phase transformation associated with it. Further discussions of linear pseudoelasticity can be found in, for example, T. W. Duerig, et al., xe2x80x9cLinear Superelasticity in Cold-Worked Nixe2x80x94Ti,xe2x80x9d Engineering Aspects of Shape Memory Alloys, pp. 414-19 (1990).
Because of the useful nature of the nickel-titanium alloy, some have attempted to change its properties to solve different design needs. For example, U.S. Pat. No. 6,106,642 to DiCarlo et al. discloses annealing Nitinol to achieve improved ductility and other mechanical properties. U.S. Pat. No. 5,876,434 to Flomenblit et al. teaches annealing and deforming Nitinol alloy to obtain different stress-strain relationships.
Binary nickel-titanium alloys have been used in the medical field. Some medical device related applications exploit the non-linear pseudoelastic capabilities of Nitinol. Examples include: U.S. Pat. Nos. 4,665,906; 5,067,957; 5,190,546; and 5,597,378 to Jervis; and U.S. Pat. Nos. 5,509,923; 5,486,183; 5,632,746; 5,720,754; and 6,004,629 to Middleman, et al.
Yet another application of nickel-titanium alloys is in an embolic protection or filtering device. Such embolic filtering devices and systems are particularly useful when performing balloon angioplasty, stenting procedures, laser angioplasty, or atherectomy in critical vessels, particularly in vessels such as the carotid arteries, where the release of embolic debris into the bloodstream can occlude the flow of oxygenated blood to the brain or other vital organs. Such an occlusion can cause devastating consequences to the patient. While the embolic protection devices and systems are particularly useful in carotid procedures, they are equally useful in conjunction with any vascular interventional procedure in which there is an embolic risk.
What has been needed and heretofore unavailable in the prior art is a medical device that exploits the benefits of linear pseudoelastic Nitinol. With the use of linear pseudoelastic Nitinol, the mechanical strength of the device is substantially greater per unit strain than a comparable device made of superelastic Nitinol. Furthermore, smaller component parts such as struts can be used because of the greater storage of energy available in a linear pseudoelastic Nitinol device.
The present invention is generally directed to cold worked nickel-titanium alloys and nickel-titanium clad alloys (nickel-titanium alloys clad with a layer of another metal) that have been deep drawn in a cold working process that produces linear pseudoelastic behavior in the alloy. The processed material may exhibit pseudoelastic behavior without a phase transformation or onset of stress-induced martensite as applied to a medical device.
In one aspect, the present invention is directed to a medical device for use in a body lumen comprising a structural element made from a cold formed nickel-titanium alloy which has been processed by plastically deforming a sheet-type product into a desired shape. Such processes may include deep drawing, pad forming, hydrodynamic forming or similar processing fundamentals commonly used in the metal forming industry. The nickel-titanium alloy remains in a martensitic phase when the structural element is stressed into a first shape and also when the stress to the structural element is relieved to assume a second shape. A sheath which at least partially envelopes the structural element in its first shape may be used to maintain a restraining force on the structural element. The sheath may be used to transport the medical device to a targeted location in the patient""s anatomy, to deploy the medical device, and/or to retrieve the medical device at the end of the procedure.
The raw Nitinol for use in the present invention has been cold formed and is further cold worked to set the desired expanded shape through deep drawing processes. The deep drawing process can be accomplished, for example, by processing a sheet of Nitinol, referred to as a blank, in a press operation that utilizes a moveable punch or mandrel and a die to stamp the blank into a particular shaped element. The structural element, can be formed into a second desired shape by etching, lasing or mechanically cutting the structural element. The structural element can be further processed, through similar cutting or shaping operations, until it is formed into a final geometry to achieve the desired medical device or component of a composite medical device. Furthermore, the cold forming and cold working from the deep drawing procedure could occur below the recrystallization temperature of the Nitinol alloy.
During its operation, the linear pseudoelastic Nitinol medical device can be stressed without developing stress-induced martensite in the alloy. Consistent with this behavior, an idealized stress-strain curve of the linear pseudoelastic Nitinol does not contain any flat stress plateaus. Furthermore, despite application of stress, the Nitinol alloy does not undergo a phase transformation from austenite to martensite or vice versa.
The present invention produces a medical device which can have greater mechanical strength at any given strain as compared to a device made of a standard superelastic Nitinol. The stress-strain curve of the present invention may also possess more energy storage capacity. As a result, for a given desired performance requirement, the present invention linear pseudoelastic Nitinol device allows for smaller struts and consequently a lower profile useful in crossing narrow lesions.
Another advantage is that because the present invention uses linear pseudoelastic Nitinol, the underlying alloy can be selected from a broader range of available materials yet still maintain consistent, mechanical properties. In other words, there is less sensitivity to material variations and processing vagaries as compared to superelastic Nitinol. In addition, since the linear pseudoelastic Nitinol has no transformation from martensite to austenite or vice versa, there is less of an influence by temperature-related effects. As a result, a medical device, such as a stent, could be cold worked via the deep drawing process or similar cold forming processing, to assume a particular shape and may be ideally suited to accept drugs and coatings which may otherwise be temperature sensitive. Additional cold forming processing includes, but is not limited to, pad forming, hydrodynamic forming, stamping and other similar processing fundamentals commonly used in the metal forming industry. Further, such a medical device could be elastically compressed into a delivery sheath at room temperature, rather than at a lower temperature normally needed for processed Nitinol, in order to place the medical device into its compressed delivery position.
There are many specific applications for the present invention including vena cava filters, septal plugs, vascular grafts, just to name a few. One specific application of the present invention is a filtering device and system for capturing embolic debris in a blood vessel created during the performance of a therapeutic interventional procedure, such as a balloon angioplasty or stenting procedure, in order to prevent the embolic debris from blocking blood vessels downstream from the interventional site. Another specific application of the present invention is a stent, including an ostium stent, which can be formed with a flange that is positioned at the ostium of a body vessel to prevent the stent from displacement once implanted in the body vessel.
In another aspect of the present invention, it is no longer necessary to fabricate the structural element forming the medical device from, as an example, a small tubing that is then heat treated to an expanded state. Rather, the structural element can start out as a large diameter tubing and assembled inward to the desired geometry to create the structure of the medical device. The specific medical geometry shape can be formed from large tubing and compressed to a smaller size for delivery to accomplish the same expanded state without need of heat treatment to the nickel-titanium material.
In another aspect of the present invention, the nickel-titanium alloy can be clad with one or more additional layers of material, such as a thin layer of gold, platinum, or palladium, to increase the radiopacity of the nickel-titanium alloy. While nickel-titanium alloy is certainly beneficial for use in medical devices, one of its shortcomings is it""s has low visibility under fluouroscopic examination. A material, such as biocompatible metal, however, can be clad over the surface of the nickel-titanium alloy to increase the radiopacity on the fluoroscoping. The nickel-titanium clad material can then be processed as described above to create a medical device or component of a composite medical device having a desired geometry. Methods for cladding or depositing one or more layers of materials onto nickel-titanium, such as electro plating, are known in the art.
In another aspect of the present invention, the material can first be annealed at a particular temperature then shaped by the deep drawing process to create a particular shape which can then be further processed (through the use of a laser or other processing means) to obtain the desired medical device geometry. Upon further processing, the particular geometry of the medical device can take on many different forms and shapes and could have many different applications in the medical field. However, the annealing process may cause this particular processed material to behave more like non-linear pseudoelastic Nitinol.
Other features and advantages of the present invention will become more apparent from the following detailed description of the invention when taken in conjunction with the accompanying exemplary drawings.